First published as an Advance Article on the web 13th August 2004 A NMR relaxation and NMR translational diffusion study is performed on a bicontinuous cubic phase of the dodecyltrimethylammonium chloride/ 2 H 2 O system with low water content. It is demonstrated how NMR relaxation measurements are combined with structural investigations of X-ray and NMR molecular diffusion measurements to create a consistent picture of the mesoscopic structure of the cubic phase. The 14 N, spin–lattice (R 1) and spin–spin relaxation rates (R 2), were measured for the surfactant at two different field strengths and at four temperatures. The water and the surfactant translational diffusion coefficients were measured by NMR field gradient technique. The experimental spin relaxation rates, and translation diffusion coefficients were interpreted in a model based on a Brownian dynamics (BD) simulation of translational diffusion along curved interfaces (P. Haansson, L. Persson and P.-O. Westlund, J. Chem. Phys., 2002, 117(19), 8634–8643, ref. 1). The model surfaces used in our BD relaxation model are also frequently used in interpretations of X-ray crystallography and in free energy calculations on bicontinuous phases. One of our BD-models was able to quantify the influence of the water/lipid interface on the NMR relaxation and diffusion data. Together with X-ray data, a consistent picture of the microstructure for this system was obtained.